Sex linked Genetic diagrams
Summary
TLDRThis educational video script delves into sex-linked disorders, focusing on how they are inherited and calculated using a Punnett square. It explains the role of sex chromosomes in determining biological sex and susceptibility to disorders. The script uses examples like red-green color blindness and hemophilia to illustrate X-linked recessive disorders, highlighting the difference in inheritance patterns between males and females. It also clarifies the distinction between genotype and phenotype, emphasizing the importance of understanding these concepts for grasping sex-linked inheritance.
Takeaways
- 🧬 Sex-linked disorders are genetic conditions that are passed down through the sex chromosomes, specifically the X chromosome.
- 🔍 Humans have 23 pairs of chromosomes, with the 23rd pair being the sex chromosomes, which include the X and Y chromosomes.
- 🚺 Females have two X chromosomes (XX), making them less susceptible to sex-linked disorders, as one X chromosome can compensate for the other in case of damage.
- 🚹 Males have one X and one Y chromosome (XY), which makes them more vulnerable to sex-linked disorders since they have only one X chromosome to carry disease alleles.
- 🟥 Red-green color blindness is an example of a common sex-linked recessive disorder, affecting the ability to distinguish certain colors.
- 🔄 Inheritance patterns of sex-linked disorders can be visualized using Punnett squares, which help predict the likelihood of offspring inheriting the condition.
- 👨👩👧👦 A carrier mother can pass on a sex-linked disorder to her children, with sons being more likely to express the disorder due to having only one X chromosome.
- 🩸 Hemophilia is another example of a sex-linked disorder, typically affecting males and being carried by females as they have two X chromosomes.
- 🧬 The script explains how to calculate the probability of offspring inheriting sex-linked disorders using genetic crosses and Punnett squares.
- 📝 When explaining sex-linked inheritance, it's important to describe the genotypes of the parents and how the inheritance of one allele from each parent affects the offspring.
Q & A
What are the two types of sex chromosomes in humans?
-The two types of sex chromosomes in humans are the X chromosome and the Y chromosome.
How do females and males differ in their sex chromosome composition?
-Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
What is the significance of having two X chromosomes in females in terms of disease susceptibility?
-Females are less susceptible to X-linked disorders because if one X chromosome is damaged, the other can compensate and help fix the problem.
Why are males more prone to X-linked disorders compared to females?
-Males are more prone to X-linked disorders because they have only one X chromosome, and if it carries a recessive disease-causing allele, they will express the disorder since they have no second X chromosome to mask it.
What is the difference between autosomes and sex chromosomes?
-Autosomes are the first 22 pairs of chromosomes that are not involved in sex determination, while sex chromosomes (gonosomes) are the 23rd pair, which include the X and Y chromosomes that determine biological sex.
What is a common example of an X-linked recessive disorder mentioned in the script?
-A common example of an X-linked recessive disorder mentioned is red-green color blindness.
How is the inheritance of X-linked disorders represented in a Punnett square?
-In a Punnett square for X-linked disorders, the alleles are represented as superscripts on the X chromosome, with a dominant allele (usually a capital letter) and a recessive allele (usually a lowercase letter).
What is the significance of the term 'carrier' in the context of X-linked disorders?
-A 'carrier' refers to an individual, typically female, who has one normal allele and one allele for an X-linked recessive disorder. They do not express the disorder but can pass the disorder allele to their offspring.
How do you determine the probability of offspring inheriting an X-linked disorder from a Punnett square?
-You determine the probability by looking at the genotypes resulting from the cross and calculating the percentage of offspring that receive the recessive allele on their single X chromosome (males) or both X chromosomes (females).
Why is it important to specify the sex when describing the phenotypes in a sex-linked inheritance scenario?
-Specifying the sex is important because the phenotype of an individual with a sex-linked disorder can differ between males and females due to the different composition of their sex chromosomes.
What is the difference between genotype and phenotype in the context of sex-linked disorders?
-Genotype refers to the specific set of alleles an individual carries for a gene, while phenotype refers to the observable characteristics or traits that result from the interaction of those alleles with the individual's environment.
Outlines
🧬 Introduction to Sex-Linked Disorders
This paragraph introduces the topic of sex-linked disorders, focusing on how they are inherited and calculated using a Punnett square. It explains the role of sex chromosomes, the X and Y, in determining biological sex and how autosomes differ from them. The paragraph emphasizes the importance of understanding chromosome structure, particularly the X and Y chromosomes, and their combinations in males and females. It also touches on the concept of females being less susceptible to certain disorders due to having two X chromosomes, which can compensate for a damaged one, unlike males who have only one X chromosome and are thus more vulnerable to X-linked disorders.
👨👩👧👦 Inheritance Patterns and Examples
The second paragraph delves into the inheritance patterns of sex-linked disorders, using red-green color blindness as an example. It describes how this condition is a recessive disorder carried on the X chromosome and represented with a lowercase 'r' as a superscript. The paragraph explains the genetic cross of an unaffected father and a carrier mother, detailing the possible outcomes for their offspring. It clarifies that unaffected sons inherit a capital 'R' from their mother, while daughters can be either unaffected or carriers, depending on whether they inherit the small 'r' from their mother. The paragraph also discusses how to represent these genetic outcomes visually and emphasizes the importance of knowing the genotypes and phenotypes for understanding sex-linked inheritance.
🧵 Calculating Sex-Linked Inheritance: Hemophilia
This paragraph shifts the focus to another common sex-linked disorder, hemophilia, to demonstrate how to calculate sex-linked inheritance. It outlines three different genetic cross scenarios with varying outcomes. The first scenario involves a male with hemophilia and a non-carrier female, resulting in 50% of offspring being carriers or non-affected. The second scenario includes a male with hemophilia and a carrier female, leading to a 25% chance for each possible genotype and phenotype among the offspring. The third scenario features a normal male and a carrier female, resulting in a 25% chance for each genotype and phenotype. The paragraph stresses the importance of accurately recording and grouping genotypes and phenotypes when calculating sex-linked inheritance.
📊 Understanding and Explaining Sex-Linked Inheritance
The fourth paragraph discusses how to explain sex-linked inheritance, particularly focusing on the differences in susceptibility between males and females. It highlights that males are more affected due to having only one X chromosome, which lacks a second X to mask the presence of a recessive disorder allele. The paragraph also addresses how to answer questions about the percentage of children with a disorder, emphasizing the need to distinguish between males and females when calculating these percentages. It provides a structured approach to explaining sex-linked inheritance, including stating parents' genotypes, explaining allele inheritance from each parent, and discussing why males are more likely to express the disorder.
👋 Conclusion and Terminology Recap
The final paragraph concludes the video script with a recap of key terminology related to sex-linked inheritance. It differentiates between autosomes, which are not sex chromosomes and are associated with autosomal diseases, and gonosomes, which are the sex chromosomes (X and Y) and are relevant to sex-linked disorders. The paragraph also distinguishes between recessive and dominant traits in the context of sex-linked disorders and clarifies the difference between phenotype (observable characteristics) and genotype (alleles on sex chromosomes). It corrects a misconception about using 'carrier' as a phenotype and ends with a reminder of the importance of understanding these concepts for grasping sex-linked inheritance.
Mindmap
Keywords
💡Sex-linked disorders
💡Gonosomes
💡Autosomes
💡Punnett square
💡Recessive disorder
💡Carrier
💡Phenotype
💡Genotype
💡Red-green color blindness
💡Hemophilia
Highlights
Introduction to sex-linked disorders and how they are inherited.
Explanation of the structure of chromosomes, specifically the sex chromosomes (X and Y).
Differentiation between autosomes and sex chromosomes in determining biological sex.
Description of the genetic makeup of males and females in terms of sex chromosomes.
Importance of understanding chromosome combinations for learning about sex-linked disorders.
Discussion on why females are less susceptible to certain disorders due to having two X chromosomes.
Explanation of how males, with one X and one Y chromosome, are more susceptible to X-linked disorders.
Introduction to X-linked disorders and their representation on the X chromosome.
Example of a sex-linked disease: red-green color blindness, its symptoms, and how it is inherited.
Demonstration of a red-green color blindness test and interpretation of results.
Genetic representation of sex-linked diseases using superscript letters on the X chromosome.
Illustration of genetic cross using a Punnett square to predict inheritance outcomes.
Explanation of the inheritance pattern in an example with an unaffected father and a carrier mother.
Discussion on how to determine the genotype and phenotype of offspring from a genetic cross.
Clarification on the difference between genotype and phenotype in the context of sex-linked disorders.
Introduction to hemophilia as another common sex-linked disorder and its inheritance pattern.
Tutorial on calculating sex-linked inheritance using hemophilia as an example.
Emphasis on the importance of accurately writing genotypes and phenotypes in genetic crosses.
Explanation of how to group and calculate the percentages of different genotypes and phenotypes in a Punnett square.
Differentiation between writing about carriers in questions versus describing phenotypes.
Guidance on answering questions that ask for the percentage of children with a sex-linked disorder.
Instructions on explaining sex-linked inheritance, including stating parents' genotypes and inheritance patterns.
Terminology recap including autosomes, gonosomes, recessive and dominant traits, phenotype, and genotype.
Transcripts
hi everybody and welcome back today
we're going to be looking at sex
linked disorders and we're going to go
through exactly how you inherit these
diseases
how to calculate them in a punnett
square and also we are going to look out
how do you explain these in longer
questions when you have to explain it
perhaps in a paragraph
so a fundamental aspect of this topic is
understanding
the structure of the chromosomes now in
the photograph in front of you
you have what we call our gonozomes
or the sex chromosomes
now when we speak about gronosomes we
are talking about the
x chromosome and the y
chromosome essentially these are the
23rd pair in humans that dictate our
biological sex
autosomes are the 22 other
chromosomes that humans have and they
are not linked to our sex determination
now in this photograph
the very large chromosome is represented
by
our x so that is the x chromosome the
smaller chromosome in blue
is the y chromosome now
depending on whether you are male or
female females will have
two x chromosomes and
males will have an x and
a y it's important to know these
combinations of by heart because it's
going to help you learn about these sex
link disorders a lot
more easily now you will notice that
especially and fundamentally females
have two
x chromosomes which makes them a lot
less susceptible to disorders because
if one of their x chromosomes is damaged
the other one can help
it out it can fix the problem in males
on the other hand you will notice that
they only have
one x chromosome and because an x and a
y
are actually not homologs in other words
they're not an actual homologous pair
they're not perfectly identical to match
um whatever is on the y actually can't
help if there's something broken
on the x now in this video i'm only
going to talk about
x linked disorders which means that
these are sex
links disorders but they're only found
on the x chromosome you do also get ones
on the y
so a very common example of a sex link
disease
is red green color blindness
now essentially red green color
blindness means that you have the
inability to see certain shades of color
and alongside here i have included a
very simple red green color blind test
if you cannot see the number in the
circle there's a possibility
that perhaps you do have color blindness
you might see the incorrect number or a
partial number
if you're hoping to know what the
correct number is it's a 2
that's hidden inside of there often
people with red green color blindness
confuse the reds the oranges and the
yellows and greens with each other
now this particular kind of sex-linked
disease
is carried on the x chromosome and it is
a recessive
disorder that means that we are going to
use a small letter to represent it but
this time we're actually going to write
it
as a superscript at the top of the x
if you don't have the disorder then your
x chromosome is going to carry a
capital r on it and that means you don't
have it
in other words in order for us to see
the variations of how this turns out
let's have a look at this example
along on the left so what we have here
is an
unaffected father and we have a
carrier mother now let's talk about the
unaffected father
if the unaffected father doesn't have
the disorder that then means that his x
chromosome
will have a capital r on it and he will
have
a y and there's nothing on the y because
this is not a y
linked disease it is an x chromosome
disease
the mother on the other hand is what we
call a carrier
what that means is that one of her
chromosomes carries it and the other
doesn't
so if we were to do her genotype she
would have one capital r
on her x chromosome and one small case
r or lowercase r that means she's
carrying that disorder
now what they've done lower is that
they've done a genetic cross like a
punnett square but with pictures this
time
and what we have is four possible
outcomes of the offspring first of all
we have an unaffected son and an
unaffected daughter now the only way
that that's possible for the unaffected
son
is that his x chromosome which he gets
from his mother
because you can only get wise from
fathers the x chromosome he got from his
mother
can only be the
capital r he can't inherit the small r
if he did
he would have the disorder and so that
then means
that the only possible thing that is on
his x chromosome
is going to be a capital r
and a y our daughter who's unaffected on
the other hand
she has two x chromosomes and now we
need to know
well what are her r's does she have two
big r's little one
what does she have well if we look at
the fact that she is not a carrier
it means that she is going to inherit
one allele from each parent and she's
going to inherit one capital letter from
her father
and she's going to inherit the other
capital letter
from her mother making her therefore
have a genotype of
a capital r on the one x and a capital r
on the other
then we look at the carrier daughter the
carrier daughter just like her mother
will have the same phenotype
she will have an x with a capital r on
it
where did she get that capital r from
she received
that one from her father because that's
all he can give to his daughters he
can't give a y
chromosome the other r on the other x
however
was inherited by and from the mother
because that's the only other place she
could have got this small r which she
carries
and so that's where her small r comes
from finally we have the affected sun
the affected sun is going to be x y
and what do we find on his x chromosome
well he inherited his y chromosome from
his father so there's nothing that we
can receive
from the dad that will tell us if
they're colorblind what's important is
where did he get that affected allele
from
and he received it from his mother so
that means the
sun will have a small letter r on his
x so to round this section off
just to ensure that we know exactly how
this works
if you are two x's
you're female you must have two small
r's on both of your x's to have the
disorder
if you are a boy you only need
one small letter r on your x
and that's because you only have one
x allele to work with on the other hand
in our dominant or capital r you can
have a girl
who has two capital rs
you can have a boy with a capital r
which means he doesn't have the disorder
and the final option oh i've made a
mistake let's just quickly rub that out
the final option is if you have
a female who is a carrier
and the female i'm just going to put it
below here because i'm running out of
space
is going to be a capital r and a
small or lower case r
now we need to look at how do you
actually calculate the sex linked
inheritance and for this one i'm going
to use hemophilia which is another very
common kind of sex-linked disorder that
we do in school
and these are three different examples
with three slightly different
outcomes it's been a little bit
challenging also to write these
h's as superscript i know that they're
written here lower
remember when we write them we want to
write the x and then we want to put it
at the top
like that uh just for reference when
you're writing this
in school so let's have a look at the
first example so our first
example is a male with hemophilia
now you are going to set out your
genetic cross like you normally do
you're going to have your geno and
phenotype of the parents
meiosis gametes fertilization and then
you're going to draw your punnett square
like you normally would you you
basically draw your normal
genetic diagram except the lettering
is just slightly altered and with this
one the question would have said a male
with hemophilia so here is our male with
hemophilia there is his one allele and
there is his other sex chromosome
and then we have a female who is a
non-carrier
sometimes they call them non-carriers
sometimes they can call a female without
hemophilia
sometimes they will also say it's just a
normal
female essentially what that means is
that this female has
two capital h's in other words she is
not a carrier of the disease and
she does not have it either
now you would do your genetic cross like
you normally would and these would be
our
outcomes now the way in which you write
your answer is
really important and so that's what i'm
going to show you now in terms of
calculating it
now what we need to do is we need to
group together our genome and phenotypes
and so looking at our punnett square we
can first of all group together our
genotypes
we're going to look at the females first
and these two females carry the same
genotypes we group them together so
that's 50 percent and then these two
male genotypes are also the same and so
we group those together also
50 the phenotype is
a reflection of that where there are 50
females without hemophilia it's
important to write that they are without
hemophilia
and that they are females this is a
sex-linked cross so you must give
the sex of the individual and then 50
males without
hemophilia now let's have a look at the
second example the second example will
have something
in the question along the lines of a
male with hemophilia
and so here are his alleles
and they are always placed together
let's not forget that
and then the next important component is
that this cross now has a female
carrier it's important to remember that
the word carrier is actually not a
descriptive word that you're allowed to
use when you talk about
the phenotype at the very end and i'll
show you exactly what i mean by that
they can use it in the question to
describe the female
but you can't use it to describe them
later on when you write out the
phenotype but i'll show you what i mean
and here is our female carrier we know
she's a carrier because she has one
capital
h on her x and one lower case h on the
other
now we are going to multiply this into
the table and let's write out our pheno
and genotype
all right now this genome phenotype is a
little bit longer because we've produced
essentially four very different
individuals and so let's have a look at
the genotype
so we're going to group all the same
genotypes together now in this
particular punnett square all the
genotypes are different this female
represents 25
this female here represents 25 this male
is 25 and this male is also 25
the phenotypes follow a very similar
distribution 25 percent of females
without hemophilia
we then have 25 females with
hemophilia it's important to note that
these 25 percent of the females with
hemophilia
won't actually be born but because a
punnett square is working out
probability
you still need to provide the
probability of a child
inheriting the disorder don't get caught
out on questions
that are asking about hemophiliac
females just because you know that
they're not actually born
but they will essentially be formed
and often a mother goes through a
miscarriage we then have 25
males without hemophilia and then 25
males with
hemophilia now let's get to the third
example
in this final one we look at a normal
male which as we can see here he has his
dominant
h on the x and the y chromosome
and then we have very importantly now a
female
carrier you will now notice that
whenever
i'm going to write about female carriers
we don't actually call them that when we
write our phenotypes but you'll see now
as i do the calculations and i write out
our genome and phenotypes you multiply
into the punnett square
and you write down your pheno and
genotype results
now this final genome and phenotype
reveals a very important way in which we
need to group our results
the genotype is very simple all we need
to do
is group them so we have 25 percent
two x's with capital dominant h's we
have an individual
a female individual who has a capital h
of a small h
25 have a capital h a dominant allele
and 25 percent of the males have a
a recessive h on their x and that's how
we record it
what i want to bring your attention to
is how you write the phenotype
50 of the females are without
hemophilia you do not write 25 percent
without
and then 25 female carrier technically
being a carrier is not a phenotype
you can't actually write that down
because it's not a physical
characteristic that can be observed
it is only a genotype i know in
questions that they will refer to them
as carriers and they have to because
that's how they're indicating that
they're heterozygous that they have a
big letter and a small letter or a
dominant and a recessive allele present
but you cannot write them that way we
then still write however the males the
same
and that we have 25 percent males
without hemophilia
and 25 males with hemophilia the last
thing that is really critical that goes
with any sex linked inheritance is
questions like the following
let's say for example three they ask you
to tell them
of the children what percentage
has hemophilia now of the four children
in example number three how many
have hemophilia now of all the children
that's boys and girls
and of all three only 25
now that refers to all the children
but what happens if the question was
what percentage of the males
have hemophilia now if we
only look at the boys we only look at
the males
and there is only one of the two males
that have it that then means that of the
boy
children 50 percent of the boy
children have hemophilia it's really
important
to read the question carefully and to
see are they asking about the children
as a whole
or are they asking about the boys or the
girls only
next we need to look at how are you
going to explain this type of
inheritance
often they will ask you how do you
explain the sex-linked inheritance
you've just seen now in the hemophilia
or
perhaps in the red green color blindness
and what you need to do
in it's a very basic structure is you're
always going to
state the mother's genotype and that
means that you need to give
what her letterings are
on her x's and her y's on her x's excuse
me
the same goes for then stating the
father's
genotype so you're going to state their
genotypes
for example let's say that our
mother's genotype is a x
with a capital h and a x
with a lower case you need to state that
and then for the father
you need to do the same let's say for
example he has a
dominant allele on his x chromosome
then what you need to do is you need to
explain how
this occurs through the following things
you are going to then explain how you
inherit one
allele from each parent that's important
males inherit only a y chromosome from
their father
and an x chromosome from their mother
females on the other hand inherit an x
chromosome from their mother
and their father now depending how long
the question is and what exactly it's
asking
they may also ask why are boys more
affected by sex-linked disorders than
girls
this is where you will then include a
section in your
answer about the fact that males only
have one x chromosome
and because they only have one x
chromosome they are more
likely to inherit the disease
why are they more likely to inherit the
disease because they do not have another
x chromosome to mask it in other words
if we look back up at this father's
genotype you'll see he only has one x
chromosome with the capital h on it the
dominant allele
if he had a recessive allele there's no
other allele that can protect him from
that disorder
the y chromosome is too small it doesn't
have enough information on it
and it can't mask anything on the x
however on this mother's genotype you'll
notice she has a dominant allele and she
has a recessive allele
which technically means she's carrying
hemophilia
but because she has two copies of the
alleles
one dominant and one recessive her
dominant allele
masks the recessive allele therefore she
does not suffer from the disorder
last but not least let's do a quick
terminology recap
we spoke about autosomes and gonozomes
autosomes are the first 22 chromosomes
and this is where you find autosomal
diseases if you've come across an
autosomal disease
you are not going to use x's and y's
you still use the standard a
and small letter a or you can even use
any other lettering depending on what
they want you to use but essentially
you only need to use your letters of the
disease you do not need
to use any other x's and y's
gronosomes on the other hand are the sex
chromosomes the x and the y and that's
what we dealt with today
you get recessive and dominant traits
and this will then depend on whether or
not the sex link disorder is a dominant
or a recessive sex-linked disorder
remember that for it to be recessive
in females you're going to need two on
each of her chromosomes so one on each
in males however they only need one
recessive allele because remember they
only have one
x in hemophilia we looked at a bleeding
disorder
that is found in males and carried by
females no female hemophiliacs
are born or exist red green color
blindness
is a type of sex lick disorder which
both males and females experience
although males are more commonly found
to have red green color blindness
and then we spoke about the phenotype
which is the physical characteristics of
the individual and what they look like
remember we can't have a carrier as a
phenotype
and lastly the genotype essentially the
alleles that we find
on their sex chromosomes thank you
everybody i hope this video
has helped you and i will see you again
soon bye
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